Charged particles beams, such as ion beams and electron beams, are used for processing work pieces in nanotechnology because charged particle beams can form very small spots. For example, focused ion beam systems are able to image, mill, deposit, and analyze with sub-micron precision. Focused ion beam systems are commercially available, for example, from FEI Company, Hillsboro, Oreg., the assignee of the present application. The ions can be used to sputter, that is, physically eject, material from a work piece to produce features, such as trenches, in the work piece. An ion beam can also be used to activate an etchant gas to enhance sputtering, or to decompose a precursor gas to deposit material near the beam impact point. An ion beam can also be used to form an image of the work piece, by collecting secondary particles ejected by the impact of the ion beam. The number of secondary particles ejected from each point on the surface is used to determine the brightness of the image at a corresponding point on the image. Focused ion beams are often used in the semiconductor industry. In one application, for example, a focused ion beams is used to cut a small trench into an integrated circuit to expose a cross section of a vertical structure for observation or measurement using an ion beam or an electron beam.
Electron beams can also be used to process a work piece. Electron beam processing is described, for example in U.S. Pat. No. 6,753,538 to Mucil et al. for “Electron Beam Processing.” Electron beams are more commonly used for forming images in a process called electron microscopy. Electron microscopy provides significantly higher resolution and greater depth of focus than optical microscopy. In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding point on the surface.
In a transmission electron microscope (TEM), a broad electron beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically thinned to a thickness of less than 100 nm. One method of preparing samples includes using a focused ion beam to cut a thin sample from a work piece, and then using the ion beam to thin the sample.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
When a charged particle beam impacts a surface, there is the potential for damage or alteration of the surface. Focused ion beam systems typically use gallium ions from liquid metal gallium ion source. Gallium ions are relatively heavy, and a gallium ion accelerated through a typical 30,000 volts will inevitably alter the work piece surface. Plasma ion systems, such as the one described in WO20050081940 of Keller et al. for a “Magnetically Enhanced, Inductively Coupled, Plasma Source for a Focused Ion Beam System” can use lighter ions, which cause less damage, but the ions will still typically alter the work piece surface. Electrons, while much lighter than ions, can also alter a work surface. When a user desires to measure a work piece with an accuracy of nanometers, changes in the work piece caused by the impact of charged particles can be significant, especially in softer materials, such as photoresist and low and ultra-low-k dielectric materials, such as polyphenylene materials.
To protect the work piece surface, it is common to apply a protective layer before charged-particle-beam processing. One method of applying a protective layer is charged-particle-beam deposition, that is, using a charged-particle beam to provide energy to decompose a gas to deposit a material on the surface. The protective layer shields the area around the cut and preserves the characteristics of the features that are to be imaged and measured. Commonly used deposition gasses include precursor compounds that decompose to deposit tungsten, platinum, gold, and carbon. For example, tungsten hexacarbonyl can be used to deposit tungsten, methylcyclopentadienyl trimethyl platinum can be used to deposit platinum, and styrene can be used to deposit carbon. Precursor gases to deposit many different materials are known in the art. The preferred material to be deposited as a protective layer depends on the application, including the composition of the underlying target surface, and the interaction between the protective layer material and the target surface.
Although charged-particle-beam-assisted deposition can locally apply a layer at the precise location where the layer is needed, applying a protective layer using charged particle beam deposition has several disadvantages. Charged-particle-beam-assisted deposition is relatively slow and, in some processes, up to sixty percent of the total processing time is consumed in deposition of the protective layer. When an ion beam is initially scanned onto the target surface to deposit material, the beam sputters material away from the surface for an initial period of time until a sufficient amount of deposition material accumulates to shield the surface from the ion beam. Even though that period of time may be small, it can be large enough to allow a significant amount of material to be removed, which causes the accuracy of the cross-sectional analysis to be compromised.
Currently, technicians quantify the dimensional change that is caused by the charged particle beam deposition of the protective layer, and then apply a correction factor to subsequent measurements to obtain an estimate of the true dimension. Such estimates are not always accurate because of the variation in the alteration by the charged particle beam. When a user desires to use an ion beam to extract a sample viewing with a TEM, as described for example, in U.S. Pat. No. 5,270,552 to Ohnishi, et al. “Method for Separating Specimen and Method for Analyzing the Specimen Separated by the Specimen Separating Method,” the user typically scans the focused ion beam in an imaging mode to locate the region of interest. The scanning causes damage to the surface. When the region of interest is located and the beam begins to mill a trench, there is additional damage to the work piece because the edges of the beam are not perfectly sharp. That is, the beam is typically Gaussian shaped, and the ions in the tail of the Gaussian distribution will damage the work piece at the edge of the trench. Damage has been found not just on fragile materials, but also on relatively hard materials.
Electron and laser beams can be used to generate secondary electrons to decompose a precursor gas to deposit a protective layer, but those beams may also damage the underlying surface—especially when they are at sufficient energy and/or current density levels for achieving favorable processing time. It is normally not practical to use such beams because deposition will typically be too slow if the beams are “weak” enough not to harm the underlying surface. Physical vapor deposition (“PVD”) sputter methods could be used to deposit protective layers in some applications, but they normally cannot be utilized for production control applications in wafer fabrication facilities because such methods cannot be used to locally apply a deposition layer onto a targeted part of the wafer surface. U.S. patent application Ser. No. 11/706,053 of Schmidt et al. for “Sputtering Coating of Protective Layer for Charged Particle Beam Processing” which is assigned to the assignees of the present invention, describes a method of PVD that can provide a localized layer. A charged-particle beam is used to sputter material from a target onto the surface. The charged-particle beam is not directed to the surface itself and damage is avoided. This method, however, is time consuming.
Another method of applying a protective coating is described is U.S. Pat. No. 6,926,935 to Arjavec et al. for “Proximity Deposition.” In this method, the charged particle beam is not directed at the area of interest, but to a region outside the area of interest. Secondary electrons decompose the precursor gas over the area of interest to provide a protective layer. As the protective layer is being created around the edge of the region of interest, the charged particle beam can be moved inward. This method is also time consuming.
Colloidal silver applied with a brush has long been used to produce a conductive protective layer in scanning electron microscopy. The silver particles used are relatively large. Using a brush to apply the layer can damage the substrate and cannot provide a localized layer.
Another method of applying a protective coating is to use a felt tip pen, such as a Sharpie brand pen from the Sanford division of Rubbermaid corporation The ink from a Sharpie pen is suitable for use in a vacuum chamber, because it dries thoroughly, and there is little outgassing in the vacuum chamber. Touching the pen to the region of interest would alter the surface, so the ink is applied near the region of interest, and the ink then wicks onto the region of interest. Compounds in the ink protect some surfaces. The area affected by the felt tip is very large compared to the sub-micron features of modern integrated circuits, and the positioning accuracy of the ink is insufficient.
The industry needs a method of rapidly and accurately applying a localized protective layer without damaging a work piece surface.